Plant hormones have been known and studied for years. Auxins, primarily indole-3-acetic acid (IAA) promote both cell division and cell elongation, and maintain apical dominance. Auxins also stimulate secondary growth in the vascular cambium, induce the formation of adventitious roots and promote fruit growth. Primary auxin activities are simulation of cell growth and elongation.
The most common naturally occurring auxin is indole-3-acetic acid (IAA). Synthetic auxins include indole-3-butyric acid (IBA); naphthalene acetic acid (NAA); 2,4-dichlorophenoxy acetic acid (2,4-D); and 2,4,5-trichlorophenoxy acetic acid (2,4,5-T or agent orange). Many of these synthetic auxins have been employed for decades as herbicides, producing accelerated and exaggerated plant growth followed by plant death.
Cytokinins, e.g., zeatin, are produced primarily in the roots of plants. Cytokinins stimulate growth of lateral buds lower on the stem, promote cell division and leaf expansion and retard plant aging. Cytokinins also enhance auxin levels by creating new growth from meristematic tissues in which auxins are synthesized.
The ratio of auxin to cytokinin can control differentiation of plant cells. Auxin is synthesized in the shoot apex, while cytokinin is synthesized mostly in the root apex. Thus, the ratio of auxin to cytokinin is normally high in the shoots, while it is low in the roots. If the ratio of auxin to cytokinin is altered by increasing the relative amount of auxin, root growth is stimulated. On the other hand, if the ratio of auxin to cytokinin is altered by increasing the relative amount of cytokinin, shoot growth is stimulated.
Plants require a certain ratio of auxin, e.g., IAA, to cytokinin for cell division. While the ratios may vary, the ratio of IAA to cytokinin must be much greater for cell division in the apical meristem tissue than the ratio in the meristem tissue of the roots. Each part of a plant may require a different IAA to cytokinin ratio for cell division. For example, different ratios may be required for cell division in the stem, fruit, grain and other plant parts. The ratio for apical meristem cell division may be considerably more, as much as 1000 times greater, than the ratio necessary for root cell division.
Shade is a key environmental cue that elicits rapid biosynthesis of auxin to promote elongated stem and petiole growth and accelerated reproduction (Tao et al. (2008) Cell 133:164). Inhibiting auxin perception can reduce shade-induced elongation of plant structures.
Auxin is also involved in plant resistance to insects and disease. In the 21-30° C. temperature range, plants produce sufficient amounts of auxins, particularly IAA, to maintain normal growth. Outside of these temperatures, auxin production declines for most plants, and microorganisms and insects are better able to multiply and feed on the plant. Pathogens can also exploit spatial auxin variations, e.g., those that can tolerate greater levels of IAA can attack the upper plant tissues, while those that require lower IAA levels attack the roots. By controlling the level of auxins in plant tissues, the ability of plants to resist attack by both pathogens and pests can be increased.
Auxin signaling is primarily mediated by three protein families: the AUXIN RESPONSE FACTOR (ARF) family of transcription factors that is responsible for the regulation of auxin-inducible gene expression, the auxin/indole-3-acetic acid (AUX/IAA) transcriptional inhibitors that interact with the ARFs and prevent their action, and F-box proteins that are part of the ubiquitin protein ligase SCFTIR1 complex and control the rapid ubiquitin-mediated degradation of the AUX/IAA in response to auxin (Leyser (2006) Curr. Biol. 16:R424). TIR1 and related F-box proteins act as auxin receptors; binding of auxin strongly enhances their interaction with AUX/IAA and ultimately leads to degradation of AUX/IAA inhibitors (Dharmasiri et al. (2005) Nature 435:441; Kepinski & Leyser (2005) Nature 435:446).
Ethylene (C2H4) is a gaseous plant hormone that affects developmental processes and fitness responses in plants, such as germination, flower and leaf senescence, fruit ripening, leaf abscission, root nodulation, programmed cell death, and responsiveness to stress and pathogen attack (Johnson and Ecker (1998) Annu Rev Genet. 32, 227-254). Another effect of ethylene on plant growth is the so-called triple response of etiolated dicotyledoneous seedlings. This response is characterized by the inhibition of hypocotyl and root cell elongation, radial swelling of the hypocotyl, and exaggerated curvature of the apical hook.
Genetic screens based on the triple response phenotype have identified more than a dozen genes involved in the ethylene response in plants. Genetically-modified and naturally-occurring mutants in ethylene-related genes are commonly used in agriculture and commercially, as well as for study of plant processes. These mutants can be divided into categories: constitutive triple response mutants (e.g., eto1, eto2 and eto3, ctr1 and ran1/ctr2); ethylene insensitive mutants (e.g., etr1, etr2, ein2, ein3, ein4, ein5, and ein6); and tissue-specific ethylene insensitive mutants (e.g., hls1, eir1, and several auxin resistant mutants). Bleecker and Kende (2000) Annu. Rev. Cell Dev. Biol. 16:1 provides a review of ethylene signaling and loss-of-function mutants.
Ethylene and auxin signaling pathways are relatively well characterized. One interaction mode occurs at the hormone biosynthesis level: Auxin induces ethylene biosynthesis by upregulation of 1-aminocyclopropane-1-carboxylate (ACC) synthase, the key enzyme in ethylene production (Abel et al. (1995) J. Biol. Chem. 270:26020). Ethylene influences auxin levels by regulating the expression of two WEAK ETHYLENE INSENSITIVE (WEI2 and WEI7) genes that encode subunits of anthranilate synthase, a rate-limiting enzyme in Trp biosynthesis (Stepanova et al. (2005) Plant Cell 16:2230), from which pathway auxin is at least partially derived (Woodward & Bartel (2005) Ann. Bot. 95:707).
Synergistic effects of auxin and ethylene have been well defined in the regulation of hypocotyl elongation (Vandenbussche et al. (2003) Plant Physiol. 133:517), root hair growth and differentiation (Pitts et al. (1998) Plant J. 16:553), apical hook formation (Li et al. (2004) Dev. Cell 7:193), root gravitropism (Buer et al. (2006) Plant Physiol. 140:1384), and root growth (Rahman et al. (2001) Plant Cell Physiol. 42:301), showing that these two signaling pathways also interact at the molecular level. Auxin and ethylene also have synergistic effects on shade avoidance. In shade, auxin and ethylene biosynthesis rapidly increases. Moreover, exogenous ethylene escalates auxin biosynthesis and addition of auxin enhances ethylene production (Abel et al. (1995) J. Biol. Chem. 270:19093; Stepanova et al. (2005) Plant Cell 17:2230).